Many applications require detection and identification of ligands, or molecules having particular binding properties. The binding properties of a particular ligand may be detected and characterized by the use of appropriate sensors. “Biosensors” have been reported in the literature and provide an alternative model for molecular screening. Biosensors are often made up of an analytical platform and a binding entity to which the ligand may bind. The detection of a ligand by a biosensor requires a ligand which binds the binding entity and an analytical platform or sensor which generates a detectable signal that can be measured. The analytical platforms measure changes in mass, capacitance, resistance, surface plasma resonance, reflectometric interference, etc. resulting from the interaction of the ligand with the binding entity.
There are particular applications for which functional biosensors would be especially useful but for which biosensors currently in the art are not ideally suited. For example, sensors could be used to screen airplane passenger luggage for trace amounts of pathogens as part of the enhanced national security effort. As another example, sensors could be used to detect pathogens in the food supply. It is estimated that 76 million Americans become ill annually due to ingesting foodborne pathogens and toxins and as many as 5,200 of these individuals will die, with an additional 325,000 being hospitalized. The U.S. Department of Agriculture estimates the cost of foodborne illness to be up to $30 billion in direct medical expenses, lost productivity, and health expenses annually (The Center for Disease Control and Prevention (CDC), News Release, 16 Sep. 1999).
There exist several methods for the general identification of target ligands, such as microbial strains, in water and food. Such methods, however, present challenges, especially when the target ligands are present in relatively small quantities. Thus, for example, many analytical techniques culture live biological target ligands so that they are present in larger, more detectable quantities prior to employing an identification method. Other methods seek to isolate the target ligands using selective binding means.
Polymerase chain reaction (PCR) methods are widely known and used for detection and identification of minute amounts of microbes in a sample. The PCR technique is effective for microbial detection; however, it presents many drawbacks and has special requirements such as a two-hour minimum run time, rigorous sample preparation, complex reactive components having limited shelf life, a precise temperature window, sophisticated PCR hardware, and highly-trained personnel.
Immuno-magnetic separation (IMS) methods seek to concentrate target ligands prior to their analysis and identification in order to provide a more sensitive means for detecting target ligands in a given specimen. In IMS methods, magnetic beads are coated with binding agents that are tailored to bind with the target ligand. The magnetic beads are then circulated within the sample so as to provide an opportunity for the target microbes to bind to their respective surfaces. Magnetic fields are then applied to separate the magnetic beads and the bound target ligands from the specimen sample. The target ligands can then be removed from the beads and analyzed. IMS systems allow the user to remotely manipulate the magnetic beads by the application of a variable magnetic field to the specimen container.
Antibodies are often used as probes and/or binding agents for the detection of target ligands due to their relatively high selectivity. Antibodies against a range of bacteria are widely available, but antibodies are relatively fragile molecules that are subject to denaturation and a consequent loss of sensitivity and other binding characteristics when exposed to unfavorable environments. In addition, the quality of antibodies can vary with production variables such as different animals. In addition, for use in biosensors, antibodies require affinity purification and stabilization, which greatly increases their cost.
Different types of analytical platforms are being used and developed for use as bases for biosensors configured to detect target microbes. Many of these sensor platforms utilize antibodies as binding agents for a target ligand. In order to identify and quantify the capture of target ligands such as pathogens or spores in antibodies or phages, various methods have been used, such as acoustic wave sensors (Pathirana et al. (2000) Biosensors & Bioelectronics 15:135-141; Xu et al. (2001) Sensors and Actuators B 75:29; Lucklum and Haauptmann (2000) Sensors and Actuators B 70:30; Wong et al. (2002) Biosensors and Bioelectronics 17:676; Babacan et al. (2000) Biosensors and Bioelectronics 15:615), micromachined cantilever force sensors (Baselt et al. (1996) J. Vac. Sci. Technol. B142:789; Raiteri et al. (2001) Sensors and Actuators B 79:115), and other sensors based on magnetic materials and fiber optics (U.S. Pat. No. 5,981,297; Chio et al. (2000) Sensors and Actuators B 68:34).
Some microscale biosensors utilize microcantilevers as analytical platforms, wherein the microcantilevers respond mechanically by deflecting and/or changing resonance frequency when matter attaches, via a specific receptor, to the cantilever's surface (see FIG. 7). One of the schemes for transducing this interaction has been through optical laser systems that resolve the nanometer scale cantilever deflections involved. Although the sensitivity of laser detection is very high, the required equipment is complicated, expensive, and occupies a significant amount of space, which is contrary to the development of compact and cost-effective detection devices. An additional drawback of optical-based measurements is the inability to effectively perform in situ solution measurements.
Other microcantilever analytical platforms utilize micro-electromechanical systems (MEMS) that operate by frequency-shift-by-mass-attachment schemes. In these MEMS systems, the microcantilever is driven by an attached oscillator circuit to exhibit oscillatory deflections at a resonance frequency. Changes in the resonance frequency of the oscillating microcantilever caused by the attachment of a target particle are then detected by circuitry attached to the microcantilever. Such MEMS-based systems, however, have several limitations: (1) each sensor requires an attached oscillator circuit and an internal power source to drive the device, (2) the sensor requires the target microbe to be brought to the surface of the sensor (and its associated circuitry and power sources) before detection can occur; and (3) these systems exhibit limited quality merit factors (Q-values) and thus exhibit less-defined resonance response peaks, especially in liquid environments (see, e.g., FIG. 24). These limitations make the detection of target microbes in low concentrations in a specimen very difficult, as it is difficult to bring the components of the entire volume of specimen into contact with the microcantilever surface. In addition, these limitations prevent MEMS systems from detecting target microbes effectively in liquid environments, such as, for example, a water supply.
The current state of the art in biosensor technology includes a number of biosensor designs. For example, U.S. Pat. No. 6,241,863 (with inventor Monbouquette) describes the development of amperometric biosensors based on redox enzymes. U.S. Pat. No. 6,239,255 describes surface plasmon resonance biosensors. Still other biosensors have been described, including biosensors which utilize functionalized microspheres for optical diffraction (U.S. Pat. No. 6,221,579), mass-sensitive biosensors (U.S. Pat. No. 6,087,187), hybrid biosensors (U.S. Pat. No. 6,051,422), metal oxide matrix biosensors (U.S. Pat. No. 5,922,183), silicon-based biosensors (U.S. Pat. No. 5,874,047), solid-supported membrane biosensors (U.S. Pat. No. 5,846,814), fiber-optic chemiluminescent biosensors (U.S. Pat. No. 5,792,621), and others.
Sensors made of magnetostrictive materials are currently used commercially to prevent the theft of high dollar value foods and merchandise (U.S. Pat. No. 6,426,700). Magnetostrictive materials produce a mechanical strain in response to a magnetic field, and also may produce a magnetic field in response to mechanical strain. This transduction/actuation scheme is both inexpensive and robust. A remote sensor for viscosity and temperature has already been developed using magnetostrictive materials (U.S. Pat. No. 6,393,921 (the “'921 patent”) and U.S. Pat. No. 6,397,661 (the “'661 patent”)). The sensor disclosed in the '661 patent is also capable of sensing an analyte using a chemically responsive outer layer comprising a hydrogel or absorbent polymer. In addition, a similar sensor has been developed for measuring pH and CO2 levels; this sensor makes use of a change in the mass of a polymer (Jain et al. (2001) Smart Materials and Structures 12:347; Cai et al. (2000) J. of Environmental Monitoring 2:556). Thus, magnetostrictive sensors are useful not only as biosensors but also are useful in a variety of other applications.
Magnetostrictive materials are generally known in the art, as are methods for evaluating and modifying their properties. Similarly, sensors comprising magnetostrictive materials are known in the art. See, for example, the '661 patent, the '921 patent, U.S. Pat. Nos. 6,579,612; 6,352,649; 6,273,965; 6,093,337; 5,821,129; 5,773,156; 5,043,693; Wing Or et al. (2003) J. Magnetism and Magnetic Materials 262: L181-L185; Na et al. (2003) J. App. Physics 93: 8501-8503; Kaniusas et al. (2003) J. Magnetism and Magnetic Materials 254-255: 624-626; Vassiliev (2002) J. Magnetism and Magnetic Materials 242-245: 66-67; Ludwig et al. (2002) J. Magnetism and Magnetic Materials 242-245: 1126-1131; Kraus et al. (2002) J. Magnetism and Magnetic Materials 242-245: 269-272; Ishiyama et al. (2002) J. Magnetism and Magnetic Materials 242-245: 1163-1165; Duenas et al. (2002) J. Magnetism and Magnetic Materials 242-245: 1132-1135; Jain et al. (2001) Applied Acoustics 62: 1001-1011; Jain et al. (2001) Smart Materials and Structures 10: 347-353; Cobeno et al. (2001) Sensors and Actuators A: Physical 91: 95-98; Chiriac et al. (2001) Sensors and Actuators A: Physical 91: 107-111; Mehnen et al. (2000) J. Magnetism and Magnetic Materials 215-216:779-781; Hristoforou (2000) Sensors and Actuators A: Physical 81: 142-146; Chiriac et al. (2000) Sensors and Actuators A: Physical 81: 166-169; Batt et al. (2000) Sensors and Actuators A: Physical 81: 170-173; Barandiaran et al. (2000) Sensors and Actuators A: Physical 81:154-157; Quandt (1997) J. Alloys and Compounds 258: 126-132; Hristoforou et al. (1997) Sensors and Actuators A: Physical 59: 84-88; Hristoforou et al. (1997) Sensors and Actuators A: Physical 59: 89-93; Atkinson and Duhaj (1996) J. Magnetism and Magnetic Materials 157-158: 156-158; Gutierrez and Barandiaran (1995) IEEE Transactions on Magnetics 31: 3146-3148; Klinger et al. (1992) IEEE Transactions on Magnetics 28: 2400-2402; Greenough et al. (1991) J. Magnetism and Magnetic Materials 101: 75-80; Hornreic et al. (1971) IEEE Transactions on Magnetics MAG7: 29; Mungle et al. (2002) Sensors and Actuators A: Physical 101: 143-149; Michelena et al. (2002) Sensors and Actuators A: Physical 100: 153-159; Cai and Grimes (2001) Sensors and Actuators B: Chemical 79: 144-149; Hristoforou et al. (1998) Sensors and Actuators A: Physical 67: 49-54; Kim (1995) Materials Science and Engineering B 34: 1-6. Thus, those of skill in the art know how to modify the composition and construction of magnetostrictive sensors to achieve the desired end result. See also, for example, Shieh et al. (2001) Progress in Materials Science 46: 461-504; Germano et al. (2000) Sensors and Actuators A: Physical 81: 134-136; Hristoforou (1997) Sensors and Actuators A: Physical 59: 183-191; Feng et al. (2003) Mechanics of Materials 35: 623-631.
However, the need remains for magnetostrictive biosensors for the detection and quantification of target ligands wherein the biosensors are small, sensitive, readily prepared, and have robust binding elements. For many applications, remotely interrogable or wireless sensors would be particularly useful. The need also exists for the development of a range of sensors that are capable of detecting very small amounts of various liquid- and foodborne pathogens that may be present in various liquids, gases, beverages, and foods during processing, storage, transportation, and marketing. In addition, a need exists for remotely interrogable, wireless sensors having an improved sensitivity for a variety of applications, including in-package monitoring of food and beverages. Such wireless sensors would be useful not only in detecting biological agents but also in evaluating environmental conditions such as, for example, temperature and humidity.